The present invention relates generally to connecting high-power radio frequency coaxial line sections. More particularly, the present invention relates to motion compensating connection fittings, used, for example, for broadcasting.
Radio-frequency (RF) broadcasting employs high power transmitting equipment that can drive transmitting antennas by way of multiple sections of coaxial line that can typically range in size, for example, from as small an inch or less in diameter to more than a foot in diameter. The sections of coaxial line can be individually on the order of twenty feet in length, terminated for example with welded-on, bolt-together flanges on the outer conductors on each end, and with pressed-in-place inner conductor joining fittings providing inner conductor continuity. Since distances between transmitters and antennas can be as much as or more than 2000 feet, it is possible to have 100 or more junctions between line sections in a full system. The connecting fittings may be required to provide structural support to the transmission line, hold the inner conductor in its location, and provide an at least moderately effective seal of the coaxial line against moisture, oxygen, pollutants, and other potential contaminants, as well as affording a low-loss bridging apparatus for transferring RF between sections.
Each joining fitting in a system is efficient to some degree at passing RF between sections. Inefficiencies at junctions may appear as heating at the joint, or may take the form of reflected RF variously returning to the transmitter or interacting with the RF that continues to come from the transmitter. The interaction can cause loss of purity in the broadcast signal, while the transmitter itself is susceptible to damage from reflected signals that reenter the output apparatus.
In the high-power RF transmission environment, each section of coaxial transmission line may be built from two concentric, substantially rigid tubes made from a high-conductivity material such as copper or a copper alloy, with the inner conductor held in place by nonconductive spacers. The spacers may be made from materials chosen for electrical, mechanical, and process properties that combine desirable minimization of effect on RF transmission with high reliability and low cost. The conductive tubes forming the coaxial line will in some instances have thermal properties similar to each other, such as their coefficients of thermal expansion, but can be subjected to unequal heat loads, such as wind chill and solar irradiance, as well as differential current density, that can cause the concentric tubes to exist at different temperatures. Over hundreds of feet, such temperature differences can manifest as length errors. These can lead to mechanical stresses manifesting as flexing of inner conductors causing deviation in impedance and can cause loci within the coaxial lines to exhibit high VSWR.
Prevention of such deviations has led to development of many designs for slip joints for inner conductors. Some such inner conductor joint designs have noticeably irregular outer surface profiles, which appear to propagating RE as lumped impedances. Joining of outer conductors using a flange that has a uniform inner surface may not minimize the overall voltage standing wave ratio (VSWR) of such a joint.
Accordingly, there is a need to provide joining apparatus for high-power RF coaxial transmission lines that at least to some extent reduces losses and reflections. The desirability of such joining apparatus is further enhanced by features that compensate for differential thermal expansion between outer conductors and inner conductors while maintaining a low-loss environment.
The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect an apparatus and method is provided that in some embodiments minimizes voltage standing wave ratio (VSWR) in a high-power RF coaxial line joint at least to some extent or avoids impedance variations associated with inner conductor motion at the joint, at least to a degree.
Some embodiments of the present invention incorporate a reactive step into the shape of a coaxial outer conductor at a joint between sections in order to compensate for a reactive step in the coaxial inner conductor at the joint. The added step introduces a compensating impedance variation.
In accordance with one embodiment of the present invention, a joining apparatus for coaxial line comprises an inner conductor junction fitting fixed to a first coaxial line inner conductor and free to move longitudinally with respect to a second coaxial line inner conductor while maintaining electrical continuity therewith; a transverse ridge component of the inner conductor junction fitting that provides spring-loaded electrical and mechanical continuity between the inner conductor junction fitting and the second coaxial line inner conductor; a first outer conductor mating flange fixed to the outer conductor of a first coaxial line section; a second outer conductor mating flange fixed to the outer conductor of a second coaxial line section; and an impedance adapting shape forming an integral part of the second outer conductor flange, wherein the impedance adapting shape introduces an impedance variation that compensates for the impedance variation caused by the transverse ridge component of the inner conductor junction fitting.
In accordance with another aspect of the present invention, an apparatus for connecting segments of high-power radio frequency signal coaxial conductor comprises means for coupling inner conductor elements of coaxial lines electrically and mechanically; means for compensating for longitudinal motion between the coupled inner conductor elements; means for electrically coupling outer conductor elements of coaxial lines; and means for compensating for diameter irregularities in the outer surface of the inner conductor coupling means with offsetting diameter alterations in the inner surface of the outer conductor coupling means.
In accordance with still another aspect of the present invention, a low-VSWR method for joining coaxial signal conductors comprises coupling the elements that comprise the inner conductor using joints that permit relative axial travel on one side of the coupling plane to afford compensation for differential expansion between inner and outer conductors; maintaining a substantially unchanging electrical profile at the site of longitudinal travel of the moving element of the inner conductor through positioning of a spring assembly to cover the end of the moving element for all positions of the moving element; coupling the elements that comprise the outer conductor using structurally robust mounting flanges indissolubly attached to the outer conductor elements being coupled; and shaping the internal profile of the outer conductor mounting flanges in such fashion that the profile introduces impedance variations that mirror and compensate for those introduced by the external profile variations of the inner conductor coupling elements.
There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.